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From: ludo@HIDDEN (Ludovic =?utf-8?Q?Court=C3=A8s?=)
To: Mark H Weaver <mhw@HIDDEN>
Subject: Re: bug#14164: letrec: detect illegal accesses to vars before
entering body
References: <877gkcuxi7.fsf@HIDDEN> <87bo9oe14d.fsf@HIDDEN>
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Content preview: Mark H Weaver <mhw@HIDDEN> skribis: > @lisp > (letrec
((a 42) > - (b (+ a 10))) > + (b (+ a 10))) ;; Illegal access > (* a b)) Please
change it to “invalid access”. Nothing illegal here. ;-) [...]
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Mark H Weaver <mhw@HIDDEN> skribis:
> @lisp
> (letrec ((a 42)
> - (b (+ a 10)))
> + (b (+ a 10))) ;; Illegal access
> (* a b))
Please change it to =E2=80=9Cinvalid access=E2=80=9D. Nothing illegal here=
. ;-)
Ludo=E2=80=99.
bug-guile@HIDDEN:bug#14164; Package guile.
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From: Mark H Weaver <mhw@HIDDEN>
To: Nikita Karetnikov <nikita@HIDDEN>
Subject: Re: bug#14164: letrec: detect illegal accesses to vars before
entering body
References: <877gkcuxi7.fsf@HIDDEN> <87bo9oe14d.fsf@HIDDEN>
<87mwt8p5u5.fsf@HIDDEN> <87wqsbd57s.fsf@HIDDEN>
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Date: Wed, 10 Apr 2013 05:25:20 -0400
In-Reply-To: <874nffobys.fsf@HIDDEN> (Nikita Karetnikov's message of
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Nikita Karetnikov <nikita@HIDDEN> writes:
>> I'll fix the manual.
>
> Thanks.
>
> So what's the difference between 'letrec' and 'letrec*', then? I fail
> to grasp it.
The difference is that in 'letrec*', the initializers are guaranteed to
be evaluated in order, and they are allowed to access earlier bindings
before entering the body. In 'letrec', the order in which the
initializers are evaluated is unspecified, and they are *not* allowed to
access earlier bindings.
> Could you add a relevant example to the manual?
The example given in the manual under 'letrec*' was basically correct.
The only problem was that it shouldn't have claimed that an error would
be reported to the user.
Here's the change I made to the manual:
--8<---------------cut here---------------start------------->8---
index 5763f36..e3a9918 100644 (file)
--- a/doc/ref/api-binding.texi
+++ b/doc/ref/api-binding.texi
@@ -218,9 +218,9 @@ variables.
@lisp
(letrec ((a 42)
- (b (+ a 10)))
+ (b (+ a 10))) ;; Illegal access
(* a b))
-@result{} ;; Error: unbound variable: a
+;; The behavior of the expression above is unspecified
(letrec* ((a 42)
(b (+ a 10)))
(* a b))
@result{} 2184
@end lisp
--8<---------------cut here---------------end--------------->8---
>> When the standard (or the manual) says "the result is unspecified", that
>> means that some value will be returned, but that value could be
>> anything.
>
> Just to clarify: When the standard says "unspecified," I can read it as
> "it's up to the particular implementation (e.g., Guile)," right?
Yes, but keep in mind that there's no guarantee that the results will be
consistent from one evaluation to the next, even within a particular
implementation. For example, the 'letrec' above could return a random
number each time it is run, or even crash randomly, though that would
clearly be undesirable and we would seek to avoid that in Guile.
Regards,
Mark
bug-guile@HIDDEN:bug#14164; Package guile.
Full text available.Received: (at 14164) by debbugs.gnu.org; 9 Apr 2013 17:24:02 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Tue Apr 09 13:24:02 2013 Received: from localhost ([127.0.0.1]:42202 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1UPcH2-0001K9-Se for submit <at> debbugs.gnu.org; Tue, 09 Apr 2013 13:24:02 -0400 Received: from li305-5.members.linode.com ([178.79.168.5]:35473 helo=cooksoni.karetnikov.org) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <nikita@HIDDEN>) id 1UPcGz-0001K0-Gy for 14164 <at> debbugs.gnu.org; Tue, 09 Apr 2013 13:23:59 -0400 From: Nikita Karetnikov <nikita@HIDDEN> To: Mark H Weaver <mhw@HIDDEN> Subject: Re: bug#14164: letrec: detect illegal accesses to vars before entering body References: <877gkcuxi7.fsf@HIDDEN> <87bo9oe14d.fsf@HIDDEN> <87mwt8p5u5.fsf@HIDDEN> <87wqsbd57s.fsf@HIDDEN> Date: Tue, 09 Apr 2013 21:22:51 +0400 In-Reply-To: <87wqsbd57s.fsf@HIDDEN> (Mark H. Weaver's message of "Tue, 09 Apr 2013 12:44:07 -0400") Message-ID: <874nffobys.fsf@HIDDEN> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha1; protocol="application/pgp-signature" X-Spam-Score: 0.8 (/) X-Debbugs-Envelope-To: 14164 Cc: 14164 <at> debbugs.gnu.org X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.13 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Sender: debbugs-submit-bounces <at> debbugs.gnu.org Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org X-Spam-Score: -0.5 (/) --=-=-= > I'll fix the manual. Thanks. So what's the difference between 'letrec' and 'letrec*', then? I fail to grasp it. Could you add a relevant example to the manual? > When the standard (or the manual) says "the result is unspecified", that > means that some value will be returned, but that value could be > anything. Just to clarify: When the standard says "unspecified," I can read it as "it's up to the particular implementation (e.g., Guile)," right? --=-=-= Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iQIcBAEBAgAGBQJRZE5vAAoJEM+IQzI9IQ38pmgP/3pdOAuqx/nyElkPbyvyOWOE 1Min18UipZk19NPMawCbISYl4UF3mnMIWSsfESs+e0yu5ht8XZ7FM0Wf5ozQDCbA oKOn3Qdwr1CWlLPz1rZsohV9Lz05S5Ly5/p/DPw0Cr1IlbLBav31Syk3Ps0Wqi+A Errb/OFOjJE5qdRqz8BDK22YFzJpRx8proW7+LpEGBNHHu5tYvRJH8pgKUdiofxw zWF25l5QZ1lkBtSr7MCAQGaWt4Cp54lx4LBt1mNNkZZ90YM9S1RZrHW+zusOkeOz n3Ml7dC+vT5MATZa86akgGRN5DMSychnp+Zk23IbhUNOuB6+nURryvUUIn1BwQBZ /NJW34aCkZYM8B3dSAVRGTJhGSXGJvJVQUasJGQ2UuX81LUk7m+hQQU/xWgJJH7j mycLoZ7DOUEUW1D/t2DD/ylSPOsDbBvpSZi+SV0CKE3Cuq8MMV0aH7uPlywZiI9B WmpO12UOHrGokbmQcMwE+645vlnsh/ZXhMvIXTGc7Uxx0LqmHd0TD1Ucn4IZpQzc TozupacoFiwiEPq1BJa3378i8Gckh+Iws1AKsofXzniz0owTPo48HH/nCRCvRGQr BaFV7nbJqjfDD9mTQNZ9P7afHhNg4661BBIEsU/2U8uEg70QqU6ujhCQR+T6eeFq hZVy9+OqZKFZVoi03DsW =D7zK -----END PGP SIGNATURE----- --=-=-=--
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From: Mark H Weaver <mhw@HIDDEN>
To: Nikita Karetnikov <nikita@HIDDEN>
Subject: Re: bug#14164: letrec: detect illegal accesses to vars before
entering body
References: <877gkcuxi7.fsf@HIDDEN> <87bo9oe14d.fsf@HIDDEN>
<87mwt8p5u5.fsf@HIDDEN>
Date: Tue, 09 Apr 2013 12:44:07 -0400
In-Reply-To: <87mwt8p5u5.fsf@HIDDEN> (Nikita Karetnikov's message of
"Tue, 09 Apr 2013 10:37:38 +0400")
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Nikita Karetnikov <nikita@HIDDEN> writes:
>>> According to the manual [1], this snippet:
>>>
>>> (letrec ((a 42)
>>> (b (+ a 10)))
>>> (* a b))
>>>
>>> should return "Error: unbound variable: a."
>
>> The manual doesn't say anything nearly that specific.
>
> Either you missed it or I misunderstood this sentence. So if you missed
> it, it can be found below 'syntax: letrec* bindings body'.
Oops! You're right, of course. Apologies for saying you were wrong.
I'll fix the manual.
>> In general, if the manual says you "may not" do something, or if the
>> R5RS says "it is an error", that means that if you do, the results are
>> unspecified.
>
> Are you talking about the result of 'letrec' or 'b'? Anyway, why does
> it return 2184 if the results are unspecified? Could you elaborate?
Well, first of all, I misspoke. In this case, I should have said (as
the R5RS did) that "it is an error", which in Scheme-standard-speak
means "anything at all could happen". This is different from "an error
is signaled" or "an error is raised" or "an exception is thrown" which
consitutes a promise to report an error.
When the standard (or the manual) says "the result is unspecified", that
means that some value will be returned, but that value could be
anything.
Although Guile includes a distinguished value shown as "*unspecified*",
which is often used in these cases, it is not always used. In practice,
we use it when we can easily do so without imposing a runtime cost, as a
debugging aid. However, it is not always used.
Thanks,
Mark
bug-guile@HIDDEN:bug#14164; Package guile.
Full text available.Received: (at 14164) by debbugs.gnu.org; 9 Apr 2013 06:38:44 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Tue Apr 09 02:38:44 2013 Received: from localhost ([127.0.0.1]:41080 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1UPSCX-0001B4-Jn for submit <at> debbugs.gnu.org; Tue, 09 Apr 2013 02:38:43 -0400 Received: from li305-5.members.linode.com ([178.79.168.5]:35465 helo=cooksoni.karetnikov.org) by debbugs.gnu.org with esmtp (Exim 4.72) (envelope-from <nikita@HIDDEN>) id 1UPSCU-0001Au-3H for 14164 <at> debbugs.gnu.org; Tue, 09 Apr 2013 02:38:39 -0400 From: Nikita Karetnikov <nikita@HIDDEN> To: Mark H Weaver <mhw@HIDDEN> Subject: Re: bug#14164: letrec: detect illegal accesses to vars before entering body References: <877gkcuxi7.fsf@HIDDEN> <87bo9oe14d.fsf@HIDDEN> Date: Tue, 09 Apr 2013 10:37:38 +0400 In-Reply-To: <87bo9oe14d.fsf@HIDDEN> (Mark H. Weaver's message of "Tue, 09 Apr 2013 01:14:58 -0400") Message-ID: <87mwt8p5u5.fsf@HIDDEN> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha1; protocol="application/pgp-signature" X-Spam-Score: 0.8 (/) X-Debbugs-Envelope-To: 14164 Cc: 14164 <at> debbugs.gnu.org X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.13 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Sender: debbugs-submit-bounces <at> debbugs.gnu.org Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org X-Spam-Score: -0.5 (/) --=-=-= >> According to the manual [1], this snippet: >> >> (letrec ((a 42) >> (b (+ a 10))) >> (* a b)) >> >> should return "Error: unbound variable: a." > The manual doesn't say anything nearly that specific. Either you missed it or I misunderstood this sentence. So if you missed it, it can be found below 'syntax: letrec* bindings body'. > In general, if the manual says you "may not" do something, or if the > R5RS says "it is an error", that means that if you do, the results are > unspecified. Are you talking about the result of 'letrec' or 'b'? Anyway, why does it return 2184 if the results are unspecified? Could you elaborate? --=-=-= Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iQIcBAEBAgAGBQJRY7c3AAoJEM+IQzI9IQ38BLYQAJ8o0caME9RdTUlXjGTx0Fx0 fMnsk9rdZ61Dyds7ifn24QXUvUNM9BFfAc6dZNTFgQm22h1aL4F+un9P5DE+KYXX 3smWLMqERkkvo/Hnl3HHFabpTGCxO1pDs3ZNl7Vl8EbD+zRa1fQUMyYuedUAi4F/ yscIJPAZphD69IpHGhpqxcpg40x5o99U37fnuARvxgtSc5tV2mPCufZeP8bZu1M2 430KQ1hH+4wyguZSnmDBdtA5fmvlDtatOaFuN1sO80+pZvpBfbxVs559aWlrQjO0 eSuBTRmkAJGs7qH74MwQaWpoti1Re+1O0RQtfIYsRKDhCuhw3IIw8YUBw0LXXk5Y ELAIKO3gV+qhT9fgQ/295zOkzt2ouCxgtVutZZTM1JbNN3NOt8fZXyvTrInOyD6/ Uu4YvJVC9l2+6eVmlrjjlOb7zvtD9qTOR3grfIr0cmkYCmRJyGaiZ0R4mslfZgSd 8f6XtQvOB+4+4G0Ah5t7Po5UbXnclVAkgMhF+H4NsPDINbhyeUUZHfLJFdTtFea3 +4+MLAtyRy5uMACG7xoW2BLDABVC4aay3ohjtTCn7GZuX1q2CC6G5uHsjV2NxfRR 9H0CuC7mAX9Cm5rX8N/2f9Ew/MwNdjEfVj175Es3ivuBtPxZhaC9vV9jG6N5Wnbf YCH2W1j2Z5IpKjGhQCrk =iSk6 -----END PGP SIGNATURE----- --=-=-=--
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From: Mark H Weaver <mhw@HIDDEN>
To: Nikita Karetnikov <nikita@HIDDEN>
Subject: Re: bug#14164: letrec: detect illegal accesses to vars before
entering body
References: <877gkcuxi7.fsf@HIDDEN>
Date: Tue, 09 Apr 2013 01:14:58 -0400
In-Reply-To: <877gkcuxi7.fsf@HIDDEN> (Nikita Karetnikov's message of
"Tue, 09 Apr 2013 08:41:04 +0400")
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Nikita Karetnikov <nikita@HIDDEN> writes:
> According to the manual [1], this snippet:
>
> (letrec ((a 42)
> (b (+ a 10)))
> (* a b))
>
> should return "Error: unbound variable: a."
The manual doesn't say anything nearly that specific. It says "Note
that while the init expressions may refer to the new variables, they may
not access their values." The R5RS says "it must be possible to
evaluate each <init> without assigning or referring to the value of any
<variable>. If this restriction is violated, then it is an error."
In general, if the manual says you "may not" do something, or if the
R5RS says "it is an error", that means that if you do, the results are
unspecified. It does not constitute a promise to report an error if you
do.
I agree that we should ideally report errors such as this, but this is
not a bug, but rather a wishlist item. Note that generating code that
detects programmer errors such as this may carry a non-trivial runtime
cost, so we might need to provide a distinct compilation mode that
inserts such debugging checks.
This is on my TODO list, but I wouldn't hold your breath :)
Regards,
Mark
bug-guile@HIDDEN:bug#14164; Package guile.
Full text available.Mark H Weaver <mhw@HIDDEN>
to control <at> debbugs.gnu.org.
Full text available.Mark H Weaver <mhw@HIDDEN>
to control <at> debbugs.gnu.org.
Full text available.
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From: Nikita Karetnikov <nikita@HIDDEN>
To: bug-guile@HIDDEN
Subject: 'letrec' allows to refer to the values of previously bound variables
Date: Tue, 09 Apr 2013 08:41:04 +0400
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--=-=-=
According to the manual [1], this snippet:
(letrec ((a 42)
(b (+ a 10)))
(* a b))
should return "Error: unbound variable: a." But it returns 2184 in
Guile 2.0.7. Either there is a bug in the manual or 'letrec' doesn't
work properly.
[1] https://gnu.org/software/guile/manual/guile.html#Local-Bindings
--=-=-=
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--=-=-=--
Nikita Karetnikov <nikita@HIDDEN>:bug-guile@HIDDEN.
Full text available.bug-guile@HIDDEN:bug#14164; Package guile.
Full text available.
GNU bug tracking system
Copyright (C) 1999 Darren O. Benham,
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1994-97 Ian Jackson.